Large quantities of sulfur are produced by the CLAUS process by reacting hydrogen sulfide originating, for example, from deacidification of natural gas or from treatments of petroleum products with the sulfur dioxide formed, for example, by combustion of H.sub.2 S or also by combustion of sulfur or sulfur-containing compounds such as pyrites, according to the reaction: EQU 2H.sub.2 S+SO.sub.2 .revreaction.3S+2H.sub.2 O
A substantial fraction of the sulfur produced by the CLAUS process is stored in the liquid state in heat-insulated tanks, then transported in liquid form to the places of use by utilizing transportation means such as tank trucks, tank barges, tank cars, or also pipelines provided with adequate heat-insulating means that allow the sulfur to remain liquid during the transportation.
The liquid sulfur obtained by the CLAUS process always contains a small amount of dissolved H.sub.2 S and hydrogen polysulfides, also called sulfanes, of the formula H.sub.2 S.sub.x wherein x represents a number equal to or more than 2. The sulfanes slowly decompose as time passes, releasing H.sub.2 S. The gaseous phase above the liquid sulfur in the storage tank or in the container used for its transportation includes, therefore, a certain proportion of H.sub.2 S, which, due to the toxicity of the gas and its tendency to spontaneous inflammation, makes the operations associated with the loading and unloading of the containers that serve for the transportation of the liquid sulfur dangerous. In order that these operations can be performed safely, the CLAUS liquid sulfur is generally subjected, prior to storage and transportation, to a gas-removal treatment, the purpose of which is to lower the content of free and combined H.sub.2 S of the liquid sulfur below a threshold that the practice has determined to be 10 ppm.
The total amount of H.sub.2 S and of sulfanes in the CLAUS liquid sulfur is generally comprised between 50 and 700 ppm and generally depends on the concentration of H.sub.2 S in the gaseous phase above the liquid sulfur and on the temperature of liquid sulfur. In addition, the relative proportions of free H.sub.2 S and sulfanes dissolved in the liquid sulfur depend also on the temperature of the liquid sulfur.
The process of removal of the H.sub.2 S present in the liquid sulfur in free and combined forms includes two phases, a first phase in which the sulfanes are decomposed according to the reaction H.sub.2 S.sub.x .fwdarw.H.sub.2 S+S.sub.x-1 and a second phase in which the released and simply dissolved H.sub.2 S and the light sulfanes are separated from the liquid sulfur.
The decomposition of the sulfanes to H.sub.2 S and sulfur is a slow reaction. The rate of removal of the H.sub.2 S and light sulfanes is limited by the speed of the decomposition reaction.
Most of the processes suggested for removing the free and combined H.sub.2 S dissolved in the liquid sulfur, are of the type in which a catalytic system formed by one or more compounds consisting of ammonia or compounds that release ammonia, or compounds having a basic character in the sense of BRONSTEDT are added to the sulfur. The catalyst system facilitates the decomposition of the sulfanes, the released H.sub.2 S being simply dissolved in the sulfur is separated from the liquid sulfur by any technique that allows the H.sub.2 S dissolved physically to escape from the liquid sulfur or to be transformed in situ to sulfur under the action of an oxidizing gas. In particular, in the process disclosed in EP-0045636, the catalytic system consists of a compound selected from inorganic compounds of phosphorus, urea, urea derivatives, dithionates, dithionites, thiosulfates, bisulfides, and bisulfites. In the processes described in U.S. Pat. No. 3,364,655, FR No. 2,159,691 and U.S. Pat. No. 4,131,437, ammonia is used as the catalyst and the H.sub.2 S released by decomposition of the sulfanes is removed by atomizing the liquid sulfur (U.S. Pat. No. 3,364,655), by stripping with an inert gas (FR-2,159,691), or by sweeping the open surface of the liquid sulfur by means of a gas such as water vapor, nitrogen, air or residual gas from a sulfur plant (U.S. Pat. No. 4,131,437). The use of a catalytic system of the type which utilizes ammonia, ammonium salts, amines and other nitrogenous compounds has also been proposed in FR No. 2,185,587 and U.S. Pat. No. 3,447,903 with transformation in situ to sulfur of the H.sub.2 S released by decomposition of the sulfanes by the action of an oxidizing gas, namely, air, in the first case and SO.sub.2 in the second case, injected into the liquid sulfur.
The above cited processes have the inconvenience of slow reaction rates which hinder carrying out the process continuously at the exit from a sulfur manufacturing process. In effect, the processes disclosed require several hours, for example, at least 2.5 hours when using a catalytic system formed by the compounds disclosed in EP-0045636, or at least from 5 to 8 hours when using ammonia as the catalytic system to obtain a liquid sulfur having a content of free and combined H.sub.2 S below the level required by the standards (10 ppm).